Fake Vision (Invisible Fog of War)

I had a game where there were places with no Fog of War but i had no wards or allies in the area so its just no Fog of War, but in this area i can't see anything so its tricking me into thinking that there's no enemies there but then random enemies would pop up out of the 'invisible Fog of War'.

How it occured: (i think) the game starts with the FoW working as usual but as i clear areas by just walking there the FoW would be permanently gone but its not really.